Showing why a system exists and who cares about which parts.
Use Case diagrams illustrate the services a system provides and the actors which care about those services. A system, when viewed through the lens of a Use Case diagram, can be thought of as a tool which provides utility/value to someone. The Use Case diagram makes that value clear.
Uses Cases are a behavior of a system. Primary Actors are the people that invoke a Use Case. Secondary actors are anyone who is involved in the behavior.
Example
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title: Use Cases
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flowchart TD
w[Writer] --> cn((Create Notes))
r[Reader] --> rn((Read Notes))
w-->rn